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A small-scale stratified downdraft gasifier coupled to a gas engine for combined heat and power production.

M. Barrio, M. Fossum#, J.E. Hustad Norwegian University of Science and Technology, Department of Thermal Energy and Hydro Power, 7491 Trondheim, Norway # Sintef Energy Research, Department of Thermal Energy, 7465 Trondheim, Norway

ABSTRACT: A small scale (30 kW thermal input) stratified downdraft gasifier is erected in the laboratory at the Norwegian University of Science and Technology (NTNU). The gasifier will be coupled to a gas engine to produce heat and power from biomass fuels. The paper describes the gasifier in detail (500 mm height and 100 mm diameter). One of the singularities of the gasifier design is that it allows for variation of the point of air injection and that air preheating is also possible. The gas engine was originally a diesel engine but it has been modified for producer gas and/or natural gas operation. These changes mainly affect the compression ratio and the fuel injection system. The paper describes the gas engine and explains the modifications. Experiments have been performed of gasification of wood pellets. The feeding rate was about 5 kg/h, giving an effect of 30 kW. The amount of air supplied to the reactor has been varied in the experiments, in addition to the location of the supply. The fuel gas composition has been measured with a GC. The amount of product gas obtained is about 12.5 Nm3/h and has a heating value of 4.9 MJ/Nm3. From these data, the power produced by the gas engine is expected to be about 5 kWe. The gas engine will be operated with mixtures of synthesis gas and natural gas and detailed measurements of cylinder pressure, compression ratio and heat released by the engine are planned in addition to emission measurements of CO, unburned hydrocarbons and NOx. The dependency of the results on the ratio of synthesis gas/natural gas will further be evaluated. INTRODUCTION The objective of this project is to produce heat and power at small scale from biomass fuels. Biomass gasification represents a competitive alternative to direct combustion for optimisation of the electricity production. Fixed bed gasifiers are known to produce a gas with low tar and particulate content, and therefore suited for small scale power production1,2. The possibility of combining producer gas and natural gas is also of particular interest and specially for Norway with large resources of natural gas. The plant consists of a stratified downdraft gasifier, a cleaning system and a gas engine. The thermal input to the gasifier is 30 kW, supplied by 5 kg/h of wood pellets. The design of the gasifier is particularly flexible. It allows the use of different gasification agents, different location of the gasification agent supply and almost every

part can be replaced. At the first phase of the project, the gasification agent will be air. This small scale reactor is suitable for laboratory work but one of its drawbacks is that the heat losses are very high compared to the amount of energy involved in the process. As a first approach, a deep bed filter will be used for gas cleaning, with sand as the filter media. Later, a more advanced high temperature filtration method will be employed3. The gas engine was originally a diesel engine that has been modified for synthesis gas or/and natural gas. A relevant feature of this project is the measurement system that has been implemented in the engine. It will allow measurement of the cylinder pressure and the crank angle, providing thus relevant information for the understanding of engine operation with producer gas. This paper describes the gasifier and the engine. The first experiences with the gasifier are also presented as well as the modifications required by the diesel engine in order to operate with producer gas and/or natural gas. SMALL SCALE STRATIFIED DOWNDRAFT GASIFICATION Downdraft gasification generally produces a low particulate and low tar gas and is therefore suited for power generation in small scale applications. There exist mainly two designs for downdraft gasifiers: the Imbert gasifier and the stratified downdraft gasifier (or open core gasifier). The Imbert gasifier is usually a cylindrical reactor with a constriction near the bottom. The gasification agent is injected just above the constriction, creating a high temperature zone. The stratified downdraft gasifier has no constriction and both feedstock and gasification agent are fed from the top of the gasifier. The air flows through the fuel bed supported by the grate at the bottom of the gasifier. Although much more popular, the Imbert gasifier presents certain disadvantages like the fuel limitations due to the constriction and the difficulties at scale up due to the radial air supply. The stratified downdraft gasifier is easier to construct and the different zones are easier to access. This gasifier has in principle better scale-up properties than Imbert gasifiers4,5,6. In the stratified downdraft gasifier four zones can be distinguished. At the top of the bed the pellets are heated and dried. Below this zone the temperatures start to be higher and the pellets release their volatile matter (devolatilization, or pyrolysis). The gaseous products from devolatilization are partially burnt with the existing air. This phenomena is called flaming pyrolysis5 and it is the source of heat for the drying and pyrolysis already mentioned as well as for the subsequent char gasification. The temperature in this zone can reach 1000-1100 °C and since it is a thin zone it has been called the "pyrolysis front" in this investigation. In the third zone, the char reduction zone, the hot gases formed in the flaming pyrolysis zone (mainly CO2, H2O) react with the remaining char in absence of oxygen at around 800-900 °C. The char is converted into the product gas mainly by the following endothermic reactions: Boudouard reaction: C + CO2 2CO Water gas reaction C + H2O H2 + CO HR = + 160,9 kJ/mol HR = + 118,4 kJ/mol

The hot gases and the char coming from the flaming pyrolysis zone provide the energy required. As these reactions proceed the temperature sinks progressively until it becomes so low (700 °C) that the reaction rates are insignificant. This means that the

extent of the char reduction zone is dependent on the amount of energy entering the reduction zone and consequently also on the heat losses from the reactor7. The bottom zone, or inert char zone, consists then of char that has not reacted because the temperatures are too low- and ash. This zone is nevertheless very important since it acts as a reservoir of charcoal that can absorb heat when conditions of operation will change and moreover acts as a particle filter. The length and position of the zones described above depend on numerous parameters that interact with each other: pyrolysis rate, gasification rate, rate of ash/inert char removal, temperature profile, heat available for reaction, fuel feeding rate, air feeding rate, heat losses, etc. It is still a challenge to understand how the zone's distribution inside a gasifier affects the quality of the product gas: gas amount, composition, tar content and particle content. This challenge has been faced through experimental work and through modelling8. Di Blasi9 has recently presented a dynamic model for stratified downdraft gasifier where finite rate kinetics for the chemical reactions are included: pyrolysis, gas-phase combustion, gas-phase water shift and heterogeneous char reactions. Another important matter is the stability of the gasifier operation; the zones can move, as observed by several researchers1,7. This fact might affect the product gas quality and this is unwanted while operating the engine. The raw pellets travel downwards inside the reactor at a rate that can be obtained from the feeding rate, assuming a constant bed height:

v pellets =

feeding rate( kg / h ) * pellets bulk density ( m 3 / kg ) reactor diameter ( m 2 )

(1)

Once the pellets have crossed the pyrolysis zone, they continue to travel downwards at the same speed (vpellets) if one assumes that the bulk density of the pyrolised pellets is the same than the one of the raw pellets1:

v pellets = v char consumed

(2)

The mechanical strength of the fuel is relevant to maintain certain porosity of the bed to prevent plugging6. In the char reduction zone, the pyrolysed pellets are partially transformed to gases through the gasification reactions. The remaining pellets (inert char) and the ashes leave the bed through the grate. This can be expressed as:

charconsumed = chargasified + charremoved

(3)

The pyrolysis front moves towards the raw pellets, as a flame front, as already suggested by Reed et al.10. Flaming pyrolysis is a fast reaction at around 1000 °C and the length of this zone is relatively small and constant. Therefore, the zones will not move if the speed of this pyrolysis front matches the velocity of the bed moving down, in other words, if the pyrolysis rate is equal to the char consumption rate.

1

The dimensions of the pyrolysed pellets are approximately 90% of those of the raw pellets.

v front = v pyrolysis - vchar consumed

where vfront vpyrolysis absolute velocity of the pyrolysis front velocity of the pyrolysis front relative to the moving bed

If the pyrolysis rate is higher than the char consumption, then the pyrolysis front moves up (vfront>0) and the length of the char reduction zone increases7. If this is the case, the flaming pyrolysis front will climb indefinitely until it reaches the top of the bed and the drying zone almost disappears. Although such operation mode is stable, the radiation from the top of such bed is a significant heat loss1. This operation mode is called "Top stabilisation mode" and has been modelled by Di Blasi9. DESCRIPTION OF THE STRATIFIED DOWNDRAFT GASIFIER The gasifier and the surrounding systems are illustrated in Fig. 1 and Fig. 2. This section gives a detailed description of each element. FEEDING SYSTEM The feeding system consists of a hopper and of two pneumatic sliding valves. This system provides a pseudo-continuous operation of the reactor. Since the two valves never open simultaneously it is a tight system. A screw feeder was tested earlier but it crutched the pellets. The feeding can be done manually or automatically. Several tests have been conducted to calibrate the feeding system, so both the weight and bulk volume of each charge of pellets is estimated. Pellets have quite uniform properties regarding size, composition and mechanical strength and are therefore better suited for small scale applications. AIR SUPPLY The pressurized air network at the Department's workshop (6 bar) supplies the air for the installation. A filter removes the eventual dust and moisture from the air and thereafter the air pressure is reduced to 3 bar. The air is then stored in a manifold that distributes it to 7 independent channels. The amount of air through each channel is measured with a rotameter and controlled with a valve. In addition, an air preheater has been installed before the manifold. Each air channel supplies air to a circular distributor around the gasifier. From each circular distributor five pipes evenly distributed supply air to the reactor. Such system guarantees an even air distribution, for any combination of channels in use. The location of the air levels is shown in Fig. 3, together with the position of temperature and pressure measurements. Contrary to the usual design where the gasifier works under atmospheric pressure, the air pressure at the reactor is slightly above atmospheric pressure, depending on the pressure losses downstream the reactor. With this design, the amount of air entering the gasifier is well controlled although there is an increased risk for leakages.

Pellets

Hopper

Window

Pneumatic valves Ignition hole and safety valve Reactor

Air distribution circular pipe

Window Air distribution circular pipe Insulator Stainless steel Glass wool Ceramic core crank Grate Gas out Solids separation chamber Air tubes

crank

Ash collector

Fig. 1 Sketch of the stratified downdraft gasifier

Pellets Product Gas Outlet Sliding Valves View port T1

T10

N2

A1 A2 A3 A4 A5 A6 A7

T2

Preheater 3 bar Reduction Valve

T3 T4 T5 T6 T7 Product Gas

T13 T8 V17

Filter Air (6 bar)

T11

T12

Ash Collector

Gas Sampling Line

Fig. 2 Sketch of the gasification unit

A1

T1

P4

A2 500 A3 230 A4 170 195 200 A5 T5 90 T6 T7 10 45 A6 A7 30 70 110 P2 P1 P0 T3 P3 265 310 T4 135 450

T2 370

Fig. 3 Location of air levels (Ai), temperature (Ti) and pressure (Pi) measurements.

REACTOR The reactor itself is a cylinder of 100 mm diameter and 500 mm height, made of refractory ceramic. It has 35 holes for air supply, 7 for temperature measurement and 4 for pressure measurement. The reactor was originally designed with a glass window, so the zones could be observed. It was possible to observe the zones through the window but it was necessary to permanently cover it due to leakages. A steel cylinder of 250 mm surrounds the reactor, with glass wool filling the space between both cylinders as insulator. The steel cylinder is also covered with approx. 7 cm of insulation. Between the feeding system and the reactor there is a view port that is used to ignite the bed and acts as well as a safety pressure release valve. The small dimensions of the reactor pose problems like for example that there is no agitator to avoid bridging, that a bed height indicator could be a disturbance and the same with the thermocouples. But on the other hand, such small diameter almost guarantees radial uniform temperatures, and uniform air distribution. The bottom section of the gasifier accelerates the gas flow so that the char and ash particles will more likely deposit at the ash collector while the gas exits the reactor. The grate is placed between the reactor and the bottom section. It is a perforated plate of 10 mm thickness, with a crank so it can be shaken manually. As experienced by other researchers, the grate is problematic. Its design has been changed several times either because the char losses were too high or because it blocked and it has been the most challenging element of the installation. Its correct operation and a controlled ash removal is vital for the stable operation of the gasifier11. When the grate blocks, the amount of char removed is zero and this also affects the velocity of the pyrolysis front, as shown earlier. MEASUREMENT EQUIPMENT The temperature inside the reactor is measured with seven thermocouples (Type K). To avoid channelling, the thermocouples are placed only 5 mm into the bed. The temperature inside the manifold is also measured, as well as at several points at the outlet pipe. The pressure in the manifold and inside the reactor is also measured with the help of pressure transducers. In order to take gas samples, a sample line has been built (Fig. 4). It consists of a steel condenser and three glass bottles in an ice bath, a moisture filter (silica gel), a pump and a gas flowmeter. A sample of 0.5 ml is taken from the gas sample line with a syringe and inserted into a gas chromatograph from SRI Instruments equipped with a TCD detector and a Supelco column (Carboxen 1000).

Product gas

To GC Flow Meter

Moisture Filter Ice Bath Water Outlet Ice Bath

Fig. 4 Gas sampling line.

SAFETY CONSIDERATIONS To allow an emergency stop, an independent pipe supplies nitrogen to the reactor. If necessary, the air flow can be rapidly stopped and substituted by nitrogen. Since there is a moderate risk for leakage, the gasifier is placed inside a room with suction system and all the control and measurement equipment is placed outside the room. CO detectors are also installed. THE ENGINE In general both diesel and Otto engines can be converted to operate on gaseous fuels. To ensure high efficiency and low emissions it is recommended that the engine is modified according to the specific combustion properties of the actual gaseous fuel, which may be very different compared to both diesel and gasoline. The diesel engine is more attractive for conversion to producer gas as the reduction in power and efficiency is less compared to an Otto engine. This is due to the higher compression ratio of the diesel cycle and also the operation conditions with high excess air ratios which reduce the difference in the volumetric energy content of diesel/air mixtures and producer gas/air mixtures. The major engine modifications for a diesel engine include reduction of the compression ratio and installation of an ignition system. The ignition system can either be a spark plug system or a system using diesel fuel in a prechamber as an ignition source for the gas. Direct injection diesel engines are more suited for producer conversion than prechamber engines due to the less heat loss to the cylinder walls, which affect the ignition of the lean producer gas. Diesel engines fuelled on producer gas are normally operated at a self aspirated mode. Contaminants in the producer gas, especially particles, can cause damages to a turbocharger. The producer gas is mixed with the intake combustion air and distributed to each cylinder by the intake manifold. For small scale integrated gasification and gas engine system the suction from the gas engine is used to feed air into the gasifier. Overheated exhaust manifolds have been reported from installations running on producer gas12. This is most likely due to the low burning velocity of producer gas/air mixtures compared to diesel or natural gas and thus a complete burnout of the gas mixture may occur in the exhaust gas channel and into the exhaust manifold. The high temperature problem can be solved by cooling of the exhaust manifold. The engine used in this project is a 3-cylinder naturally aspirated direct injected diesel engine (Zetor Z4901). The table below shows the original specifications of the diesel engine: Table 1 Original engine specifications.

Concept Number of cylinders Displaced volume (dm3) Bore (mm) Stroke(mm) Compression ratio Power @ 2200 rpm (kW) Torque @ 1500 rpm (Nm) Specific fuel consumption (g/kWh) 3 2.7 102 110 17 34.2 150 245 + 10%

The engine was built in 1980, and was originally design for standby stationary power production so instead of an airblown radiator for cooling it is equipped with a water jacket heat exchanger, common in marine applications. The heat exchanger is cooled with cold water being then possible to quantify the amount of heat rejected from the engine. The engine's exhaust manifold is also cooled to avoid problems with overheating, as mentioned before. The initial phase of experiments will include a mapping of the engine characteristics using natural gas as a fuel. These data will be the basis for comparison and discussion of the results for different gas mixtures. The next step will be to use synthetic producer gas as a fuel as the most extreme low value gaseous fuel. Further experiments will include mixtures of synthetic producer gas and natural gas. COMPRESSION RATIO, IGNITION SYSTEM AND FUELLING SYSTEM The original compression ratio is too high for producer gas operation. Our intention is to operate the engine with a variety of gases, from natural gas to producer gas and thus the compression ratio is reduced down to 11:1.13 The engine has separate cylinder heads, what makes the modifications easier. The fuel injectors are replaced by spark plugs of the type used in small motorcycle engines. The small spark plugs are used to minimise the thermal mass of the spark plug, thus reducing the cooling effect and the possibility of flame quenching in the ignition zone. Each spark plug is connected to a high voltage coil that provides sufficient ignition energy. The diesel pump is removed, and an optical pickup for the electronic ignition system is mounted in its shaft. The rotational speed of this shaft is ½ of the engine speed, which is needed for the ignition system. The ignition timing can be accurately set with this system. A standard fuel feeding system is mounted and modified in order to be able to operate natural gas, producer gas and mixtures of gases. MEASUREMENT EQUIPMENT A pressure sensor from Optrand Inc. specially designed for cylinder pressure measurements has been installed in one of the cylinders. In addition, a crank angle decoder has been installed which gives a step signal twice per degree. These pulses trigger the pressure measurements and with this system the cylinder pressure versus the crank angle is recorded 720 times per revolution. The decoder also generates a one step signal for every other revolution in order to identify the compression stroke of the 4-stroke cycle of the engine. The time dependent volume of the cylinder is given explicit from the crank angle and the geometry of the cylinder, connecting rod length and crank radius. Thus, indicated values for power, efficiency, mean effective pressure and rate of heat release can be found. The air flow to the engine is calculated from the pressure drop over a "honeycomb" viscous duct meter. The heat balance for the engine is calculated from measurements of the flow of water through the heat exchanger and the according inlet and exit water temperatures. In addition the temperature of the fuel gas/air mixture and the exhaust gas temperatures are measured using thermocouples. Standard operation will include measurements of the oxygen concentration in the exhaust gas which combined with the measured inlet air flow can be used to estimate

the excess air ratio. More detailed analysis of the exhaust gas will include measurements of CO, CO2, UHC and NOx concentrations. All output signals from temperature sensors and flow controllers are sampled with a commercial acquisition system connected to a PC and data are presented on the screen using a LabView set-up. For the measurements of the cylinder pressure a high speed data acquisition system is installed which also is connected to the PC. To measure the power generated by the engine, a hydraulic brake has been installed. A load cell attached to the brake measures the torque. GAS CLEANING As a first approach deep bed filtration in planned, using sand as the filtration media. The pressure loss across the filter will be observed and when necessary, the sand will be renewed. Eventually, a pump will be installed after the filter to avoid high overpressure inside the gasifier. The engine will have a suction effect later on. Later, a more complex filtration system at high temperature will be introduced3. PRELIMINARY GASIFICATION EXPERIMENTS Several experiments have been conducted varying the amount of air supplied and its distribution and varying the preheating conditions. These parameters affect: (1) (2) (3) (4) (5) (6) (7) Fuel/air ratio Temperature profile inside the gasifier Position of the zones Stability of the gasification process Product gas quality Product gas composition Amount of product gas.

It has been found through the experiments that the reactor needs considerable preheating. If the reactor is cold, it absorbs a large amount of the heat produced and the gasification process can hardly be established. All experiments include thus approx. 12 hours of preheating at around 200 °C. After preheating, a small amount of charcoal (small pieces) was sent through the ignition port and thereafter some pieces glowing charcoal. When the temperature starts to rise, the ignition port is closed and the reactor is filled with pellets. The air flow is adjusted to the desired amount and the experiment starts. The pellets have a diameter of 8 mm and a length that varies between 5 and 15 mm. The reactor does not have a bed height indicator. Nevertheless, an alternative simple method has been found to measure and control the fuel level. The bed height is controlled by observing the temperature at the top of the gasifier (T1). When the gasifier is filled with pellets, a new charge of pellets only affects T1, that decreases suddenly below 100 °C. When the temperature T1 stabilises at around 180 °C depending on the preheating temperature-, the level is below T1 and a new charge is fed. If other temperatures are affected by the new charge of pellets, then the bed height is lower. In most experiments, the reactor has been kept filled with pellets. In this way, one possible parameter, bed height, is eliminated and also the heat produced in the pyrolysis zone can be absorbed by the raw pellets as well as by the char bed and not radiated to the top of the reactor.

Fig. 5 shows the temperature record from one of the experiments. The bed height has been kept around T1. The figure shows how the pyrolysis front moves up. This has been observed in all the experiments although the speed of the front has varied. It has been found that several operational parameters like the velocity of the front, the feeding rate and the air excess ratio depend on the temperature of the char reduction zone (Fig. 6).

1200

T6 1000

800

T7

T (°C)

600 T5 400 T4 T3 T3

200 T2 T1 0 0 10 20 30 40 50 time (min) 60 70 80 90 100

Fig. 5 Temperature record of one experiment.

25 V front (mm/min) V pellets(mm/min) 20 V py roly sis (mm/min) lambda *10 990 Amount of air (Nm3/h) T py roly sis (°C ) 980 970 Various units 15 960 950 10 940 930 5 920 910 0 820 830 840 850 860 870 880 890 900 900 910 T py roly sis (°C ) 1000

T gasific ation (°C )

Fig. 6 Influence of the temperature of the char reduction zone on the gasifier operation. The fuel/air ratio is not considered a parameter in this discussion. The pellets were supplied so the bed height was constant. This means that the fuel/air ratio is not a parameter since it is not the operator who has control over it but the gasifier, as already shown by other authors14.

The gasifier has been operated with several air distributions: (1) 100% supply from level A1 ("traditional" open core gasifier) (2) 80% supply from level A1 and 20% supply from level A4 (3) 20% supply from level A1 and 80% supply from level A4. The air excess ratio obtained for distributions (1) and (2) is about 0.4-0.45 while the distribution (3) has given a lower ratio (0.3). Distribution (3) also shows some differences on temperature profile like higher char gasification temperature. Fig. 7 shows the temperature profiles during two experiments: one with distribution (1) and another with distribution (3) but both with approximately the same amount of air supplied. Profiles 1a, 1b and 1c correspond to the same experiment, but with the pyrolysis front at different locations. The figure shows that the heat created during the flaming pyrolysis is used in a more efficient way with air distribution (3). The temperature profiles also show that the temperature below the pyrolysis front is always above 800 °C, i.e. there is no inert char zone. This can mean that the length of the char reduction zone is too short and therefore the gasification reaction is uncompleted.

50 0

1a 1b

45 0

1c 3 air inlet

40 0

35 0

30 0 He ight (m m )

25 0

20 0

15 0

10 0

50

0 0 20 0 40 0 60 0 80 0 10 00 12 00 T(°C )

Fig. 7 Temperature profile inside the reactor. The behaviour of the gasifier suggests that the reactor will operate satisfactory in "Top stabilisation mode", partially because of the low moisture content of the pellets. This mode of operation was tested in one of the experiments, as shown in Fig. 8. Top stabilisation mode was reached after 200 minutes of operation. One can observe the large variations in the temperature at the top of the bed, as a result from the semi-continuous feeding. Pyrolysis of fresh pellets alternates with char

combustion at the top of the bed and this periodic behaviour affects the temperatures along the reactor, disturbing therefore the stable gas production. The height of the bed is extremely difficult to control with this mode of operation. In addition, the large and frequent temperature variations can damage the thermocouples.

1200

1000

800 T (°C )

600

400

T6 T4 T3 T2

200

T1

0 0 60 120 t (min) 180 240

Fig. 8 Top stabilisation mode. Temperature record. Therefore, the conclusion from this experiment is that top-stabilisation mode, although feasible, is not suitable for our reactor. Under this mode of operation, the semicontinuous feeding becomes an inconvenience. Further tests have been conducted having the height of the pellet's bed above the point where the air is injected. This mode of operation has shown to be stable and more experiments will be conducted soon. The approximate gas composition of the product gas is shown in Table 2. The amount of gas produced is calculated from the N2 content of the product gas and the amount of air provided. Table 2 Product gas composition, heating value and amount produced. Product gas composition N2 (% vol.) CO (% vol.) CO2 (% vol.) O2 (% vol.) CH4 (% vol.) H2 (by difference) (% vol.) Heating value (MJ/Nm3) Amount of product gas (Nm3/h) CONCLUSIONS (1) A small scale stratified downdraft gasifier is erected at our laboratory. The thermal input is 30 kW, supplied by 5 kg/h of wood pellets.

46.8 20.6 10.2 1.5 0.0 20.9 4.9 12.6

(2) A gas engine is modified so it can operate with the product gas and with mixtures of product gas and natural gas. The modifications affect the compression ratio, the ignition system and the fuelling system. (3) The preliminary experiments show that the pyrolysis front moves up; the velocity of the front depends on the char reduction zone temperature. In order to achieve stability, this temperature should be as high as possible. The air distribution can be altered so the air supplied is more efficiently use for heat production. The amount of char removed also affects the stability of the process. (4) Top-stabilisation mode, although feasible, is not suitable for our reactor. Under this mode of operation, the semi-continuous feeding becomes a problem. It is nevertheless possible to reach stable operation by keeping the bed height above the air injection point. (5) The air distribution seems to affect the air excess ratio. (6) Based on the preliminary experiments, the gasifier produces about 12 Nm3/h of product gas. This gas has a calorific value of approx. 5 MJ/Nm3 and contains approx. 20 %vol. CO and 20 %vol. H2. ACKNOWLEDMENTS This project is financed by the Norwegian Research Council. The authors want to thank the enthusiastic cooperation of Ole Birger Svendsgaard, Torsten Goehler and Uk Chantaka. REFERENCES Reed, T.B. & Gaur, S. (1999). A survey of biomass gasification 2000. The Biomass Energy Foundation, Inc., 1810 Smith Rd., Golden, CO. 80401. 2. García-Bacaicoa, P. , Bilbao, R. & Usón, C. (1995). Air gasification of lignocellulosic biomass for power generation in Spain: Commercial plants, Proceedings of the 2nd Biomass Conference of the Americas, USA, pp. 676-694. 3. Risnes, H. & Sønju, O.K. Evaluation of granular bed filtration for high temperature applications. This conference. 4. Xu, M., Gu, Z.Z., Sun, L., Guo, D.Y. & Han, T. (1997). Research on straw waste gasification and application in straw pulp mill, Developments in Thermochemical Biomass Conversion, Blackie Academic & Professional, 1997, pp 892-899. 5. Reed, T.B. & Das, A. (1988). Handbook of biomass downdraft gasifier engine systems, The Biomass Energy Foundation Press, 1810 Smith Rd., Golden, CO. 80401. 6. Schenk, E.P. , van Doorn, J. & Kiel, J.H.A. (1997). Biomass gasification research in fixed bed and fluidised bed reactors, Gasification and Pyrolysis of Biomass, Stuttgart, April 1997. 7. Milligan, J.B. , Evans, G.D. & Bridgwater, A.V. (1993). Results from a transparent open-core downdraft gasifier, Advances in Thermochemical Biomass Conversion, Blackie Academic & Professional, 1993, pp. 175-185. 8. Reed, T.B. & Levie, B. (1984). A simplified model of the stratified downdraft gasifier, International Bio-Energy Directory and Handbook, 1984, pp.379-390. 9. Di Blasi, C. (2000). Dynamic behaviour of stratified downdraft gasifiers, Chemical Engineering Science, Vol. 55, No. 15, pp. 2931-2947. 10. Reed, T.B. & Markson, M. (1985). Biomass gasification reaction velocities, Proceedings of FPRS industrial wood energy forum '83, No.7, Vol. 2, pp. 355-365. 1.

11. Gabroski, M.S. & Brogan, T.R. (1985). Development of a downdraft modular skid mounted biomass/waste gasification system. 12. Hansen, U. et al. (1996). Heat and power from small scale biomass plants in rural regions. A: typical application case study in Mecklenburg-Vorpommern. Proceedings of the 9th European Bioenergy Conference, pp. 1318-1323. 13. ImechE (1996). Using Natural gas in engines, Seminar Publication. 14. García-Bacaicoa, P. , Bilbao, R., Arauzo, J. & Salvador, M.L. (1994). Scale-up of downdraft moving bed gasifiers(25-300 kg/h) - design, experimental aspects and results, Bioresource Technology 48 (1994), pp. 229-235.

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